A printing ink is generally formulated according to strict performance requirements demanded by the intended market application and desired properties. Whether formulated for office printing or for production printing, a particular ink is expected to produce images that are robust and durable under stress conditions.
In a typical design of a piezoelectric ink-jet printing device, the image is applied by jetting appropriately colored inks during four to six rotations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head, i.e., there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
Hot-melt inks typically used with ink-jet printers have a wax-based ink vehicle, such as a crystalline wax. Such solid ink-jet inks provide vivid color images. In conventional ink systems, crystalline-wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 120° C. to 140° C. The wax-based inks are heated to such high temperatures to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is at approximately 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium (such as paper), the image comprised of wax-based ink is pressed into the paper.
However, the use of crystalline waxes places limitations on the printing process used for conventional solid inks. For example, the printhead is kept at greater than 120° C. during the printing process which may lead to a number of problems. At these high temperatures, dyes that are molecularly dissolved in the ink vehicle are often susceptible to unwanted interactions leading to poor ink performance, for example: photo-oxidation from light (resulting in inferior lightfastness); thermal degradation (e.g., fluorescent dyes can lose fluorescence when the ink is heated to a temperature greater than 120° C.); dye diffusion from the ink into paper or other substrates (leading to poor image quality and showthrough); leaching of the dye into other solvents making contact with the image (leading to poor water-/solvent-fastness). Moreover, when the printhead is cooled and re-warmed, the resulting contraction and expansion of the ink requires a purge cycle to achieve optimum printhead performance. Enhanced mechanical robustness of the final printed image is also desired.
While conventional ink compositions, such as Color Qube™ 9200 Series solid ink, are used successfully, there is a need for a new type of phase-change ink capable of being printed via the piezo ink-jet printing processes. Furthermore, there is a need for ink compositions that can be processed at lower temperatures and with lower energy consumption having improved robustness, improved jetting reliability and latitude, and improved transfuse properties and can be fixed at low pressures. In addition, there is a need for phase-change ink compositions that exhibit desirably low viscosity values at jetting temperatures, generate images with improved look and feel characteristics, generate images with improved hardness and toughness characteristics, and are suitable for a number of commonly used substrates.
Furthermore, it is desirable to ensure that migration, evaporation or extraction of toxic or otherwise hazardous compounds used in such inks are reduced or eliminated. When such inks are used in certain applications (such as food packaging and direct to paper printing), it is desirable to reduce or eliminate the amount of unreacted monomers present, which may migrate or transfer from an image, in order to meet environmental, health, and safety requirements.
U.S. Pat. No. 6,896,937 discloses a radiation-curable hot melt ink composition comprising a colorant, a polymerizable monomer and a photoinitiating system comprising 0.5% to 1.5% by weight of an aromatic ketone photoinitiator, 2-10% by weight of an amine synergist, 3-8% by weight of a second photoinitiator different than the aromatic ketone photoinitiator and capable of undergoing alpha cleavage, and 0.5%-1.5% by weight of a photosensitizer. U.S. Pat. No. 6,896,937 also discloses liquid curable ink compositions and compositions with liquid diluents, these inks are not solids at room temperature. Furthermore, U.S. Pat. No. 7,322,688 discloses a method of inkjet printing curable inks but these inks are polymerized by a cationic photoinitiating system.
However, the conventional curable ink vehicles of the above references do not contain the ethoxylated octylphenol derivatives of the present disclosure and contain diluents. The cured hardness of the inks formulated by the above publications was also found to be less than 66 with a curing rate of about 100, less than half the curing rate of the inks of the present disclosure.
In U.S. application Ser. No. 12/642,538, filed Dec. 18, 2009, entitled “Curable Solid Overcoat Compositions”), which is hereby incorporated by reference herein in its entirety, describes low shrinkage radiation curable solid ink compositions containing commercial Licowax resins were disclosed as an alternative to conventional solid inks which showed improved performance over the inks disclosed in the above publications. These inks are acceptable for most applications, but there is a need to enhance their curing properties for increased printing speeds or reduced energy usage (green inks); for improved jettability and lower jetting temperature; and for reducing the percentage shrinkage seen upon cooling from jetting temperature to below crystallization temperature.